Thermo-stable coating die design method and apparatus

ABSTRACT

A method of designing and the resulting thermally stable heated coating die apparatus, the die apparatus including a die having a die geometry and a heating system with heaters and temperature sensors. The method and resultant apparatus provides minimized temperature gradients, flat die lip faces in a die to roll plane and a flat die in a plane perpendicular to die flat lip faces and parallel to substrate width. The method optimizes simultaneously: die geometry, placement of the heaters, placement of temperature sensors, and shielding from operating conditions, using heat transfer and structural numerical modeling and statistical analysis while considering die functionality characteristics, minimum increment of temperature measurement and control accuracy related to minimum acceptable deviation from flatness, coating die material of construction relative to thermo-structural material properties, and desirable coating die material properties.

CROSS-REFERENCE TO RELATED APPLICATION(S)

The present application is a divisional application of, and claimspriority to and the benefit of, pending U.S. patent application Ser. No.10/449,308 filed May 30, 2003.

BACKGROUND OF THE INVENTION

The present invention relates to the field of coating dies and, inparticular, to a methodology for the design of heated coating dies whichare capable of maintaining dimensional flatness of its coating lips atoperating temperature under actual operating conditions.

A heated coating die is typically used to coat molten polymer containingmaterials, such as adhesives and other coatings (collectively“coatings”). These coatings are fed into the coating die, whichdistributes them across its width. Pressure forces the coating fluidthrough a feed gap formed in the die. The exiting point of the gap isreferred to as the coating lips. In many coating applications, the lipfaces form a film on the substrate at the lip faces. This film formingregion is referred to as the coating bead. In order for the finalcoating to be uniform across the width of the coating lips (and thus thecoating), the coating lips and substrate need to form an even gap(assuming the distribution within the die is uniform).

Lip face flatness measurements on commercially available heated coatingdies indicate that the lip surfaces when heated are far from flat.Though the coating lips may be ground to better than 0.001″ when cold,the state of the die when heated can be bent several thousandths of aninch. This does not lend itself to a robust coating process. Three knownmethods of managing the bending state are:

(1) Attempt to bend the die in the opposite direction mechanically,typically by using adjustments associated with the die station.

(2) Machining the coating lips flat while the die is heated as part ofthe fabrication of the die.

(3) Pushing the coating lips and substrate into a soft rubber roller,then use feed gap adjustments to redistribute the coating fluid tocounteract the uneven flow resistances across the lip.

Though all these methods are in use, none of them lead to a sufficientlycontrolled and robust process.

In the first method, the loss of precision in the die is transferred viamechanical forces to another device (i.e., die station), which thenloses its precision. Additionally, internal stresses which cause thebending are not eliminated, but rather shifted. Finally, once coatingstarts, the bending state can change due to interaction of the die'sheating system and the flow of coating fluid, making the initialadjustment ineffective.

The second method also develops problems. First, even if the die can bemachined while heated, when the die is cold it will be bent in theopposite direction which creates uncertanties in its mounting to the diestation. Additionally, once coating starts, the bending state may changeleading to the machined surface no longer being flat. Further, there isuncertainty as to how flat a die can be machined while hot.

The third method is highly non-linear and can lead to long unstablestart-ups of the production line. It can also lead to defects in thecoating, which may not be discovered in a timely manner.

All three of these methods suffer from the difficulty in determining theinitial hot gap between the coating lips and substrate. In a commonmethodology a light is shone through the gap between the lip face andsubstrate (or back-up roll), and the die is visually adjusted to beparallel. If the evenness of this gap changes significantly at start-updue to the interaction of the heating system and flow of coating fluidleading to a temperature redistribution within the die and thus thebending state changing, another uncertainty is thereby added to theprocess.

A need therefore exists for a robust, quick start-up coating process,which is stable before and during coating, and in which the bendingstate is controllable. The present invention provides a solution to meetsuch need.

SUMMARY OF THE INVENTION

In accordance with the present invention a method is provided fordesigning the die geometry, its heating system and temperature sensorslocation in such a way that the normal state of the coating lips is flat(whether hot or cold, whether coating or not). Further, in accordancewith the present invention exemplary die apparatus implementing suchdesign method is provided. Accordingly, non-precise methods ofmechanically adjusting bending and uncertain machining methods becomenot needed and the confounding of the bending state with other variablesaffecting coat weight variation is eliminated.

In accordance with the present invention a coating die apparatus isprovided which includes:

a die having a rear portion, a width and at least two coating lips at afront portion distal from the rear portion, the at least two coatinglips spanning across the width and adapted to provide at least onecoating gap between the at least two coating lips and a substrate uponwhich a fluid layer is applied onto the substrate from between the twocoating lips and across the width; and

an integrated heating system coupled to the die to monitor and controltemperature in such a way as to minimize temperature gradients bothacross the width (cross-width) and front to back and top to bottom(cross-section).

The integrated heating system can further include groups of cross-widthheaters spaced within the die in back portion to front portion directionand/or front portion to back portion direction, across the width inzones. Each zone has a respective cross-width temperature sensor. Eachcross-width temperature sensor is coupled to a respective cross-widthtemperature control system to regulate heat being applied by therespective cross-width heaters in the respective zone.

The integrated heating system can further include one or morecross-section heaters spaced within the die longitudinally across thewidth. Each cross-section heater has a respective cross-sectiontemperature sensor. Each cross-section temperature sensor is coupled toa cross-section temperature control system to regulate the heat beingapplied by the respective cross-section heaters.

In accordance with the present invention, a thermally stable coating diemay contain a heating system composed of cartridge heaters andtemperature sensors for heating control, and is designed to maintain itsdimensional flatness to within specified tolerances in Y-Z and X-Zplanes by minimizing temperature gradients across the width in the X-Yplane and/or compensating where gradients are difficult to remove bycreating counter-balanced temperature gradients. Flatness of the die maybe purposefully altered by unbalancing the heating system in acontrolled manner. Heater and temperature sensor placement are optimallydetermined using finite element modeling and/or measurement and/or othermethodology to calculate and/or determine temperatures and/ortemperature-distribution and/or the resulting thermal distortions in thedie and utilizing an optimization procedure. Heat flux, stress, orstrain measurement techniques or sensors, as well as statisticalanalysis can be utilized.

The heated (or unheated) die to which the present invention can beapplied consists normally of 2 to 3 sections. In the case of twosections, a single feed gap is created, producing a single layercoating. In the case of a three section die, two feed gaps are createdproducing a two layered coating. Those skilled in the art can appreciatethat potentially multiple layers could be added.

The geometry of the die, the heater placement and temperature sensorplacement are optimized in such a way that upon heating, the intrinsicstate of the die results in the lip faces being flat relative to thesubstrate. This is accomplished by first simplifying the die geometry,removing unneeded material (usually steel) which leads to hot/coldspots. Next, the geometry of the die is designed in such a way that allportions of the die which remain are amenable to being heated and/orinsulated from heat loss and temperature monitored. Next, heaters areplaced in such a manner as to allow uniform heating of the entire die.Next, temperature sensors are placed in locations which accuratelyindicate the temperature state of the heater zones which they monitor.All the above may be verified and optimized by calculation using anumerical heat transfer model. The thermal deformation can be estimatedby mapping the temperature results onto a numerical structural model.The thermal and structural models are run to account for processvariations—fluid flowing through the die, no fluid flowing, etc. Onceall the parameters (die geometry, heater placement, temperature sensorplacement) are optimized and a design has been developed iteratively,the die is fabricated. Note that in addition to the thermal andstructural requirements any changes to die geometry need to occur withina design window which leads to a die which is still functional to itsintended purpose (i.e., coating a fluid onto a substrate). Afterfabrication and verification of flat lip faces when the die is cold, thedie is heated and flatness of the lip faces are measured hot. Smallchanges to temperature setpoints are made to adjust the heating systemto bring the die flat. These set point offsets may be verified in thedie station, and adjusted if needed. The temperature sensors and controlsystem used which can provide the smallest measurable/controllableincrement of temperature results in a correspondingly minimum change inbending state.

This invention can also be applied to normally unheated coating dies bylocally heating/cooling to control the bending state. Additionally,those skilled in the art can appreciate that the practice of the presentinvention can be applied to other die types, i.e., extrusion dies,curtain dies, etc.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a shows in simplified form a commonly known single layer coatingoperation.

FIG. 1 b depicts various planes associated with the implementation ofthe present invention.

FIGS. 2 a and 2 b show respectively in simplified form both a singlelayer and dual layer coating operation.

FIG. 3 shows in block diagram form the design process in accordance withthe present invention.

FIG. 4 shows in block diagram form the “develop new design concepts”aspect of the design process in accordance with the present invention.

FIG. 5 shows in block diagram form the “run sensitivity studies” aspectof the design process in accordance with the present invention.

FIG. 6 shows a graph of bending magnitudes for different heatingconfigurations.

FIG. 7 shows an exemplative solid model with heaters in place.

FIG. 8 shows an initial thermal mapping cross section of a die.

FIG. 9 shows a subsequent thermal mapping cross section of an improveddie in accordance with the present invention.

FIG. 10 shows schematically a simplified die apparatus in accordancewith the present invention.

FIGS. 11 a-11 e show in representative X-Y plane cross-section variousdie apparatus embodiments in accordance with the present invention.

FIG. 12 depicts a portion of a die apparatus embodiment in accordancewith the present invention in conjunction with the operation of itscross-width heating and control system.

FIG. 13 depicts a portion of a die apparatus embodiment in accordancewith the present invention in conjunction with the operation of itscross-section heating and control system.

FIG. 14 shows a further portion of a die apparatus embodiment inaccordance with the present invention in conjunction with furtherattachments affecting overall temperature distribution within the die.

FIGS. 15 a and 15 b show in simplified schematic cross-section view aportion of a die apparatus embodiment in accordance with the presentinvention in conjunction with still further attachments affectingoverall temperature of the die.

DETAILED DESCRIPTION

Referring to FIG. 1 a, a commonly known coating technique for asingle-layer coating is shown in simplified form. Liquid to be coated ina single layer on the substrate is fed past an elongated slot formed ina die (thus, this technique is also sometimes referred to as “slotcoating”). The slot is positioned at approximately a right angle to thedirection of travel of the substrate. The die is stationary, but thehead of the die, having two coating lips which define the opening of theslot, are placed adjacent to the substrate. A substrate may travelaround a back-up roll as it passes in front of the coating lips. Theslot formed by the coating lips and the substrate have substantiallyequal widths, such that the entire cross substrate width of thesubstrate is coated in one pass by the fluid as it flows out of the dieand onto the moving substrate. X, Y, Z coordinate system 23 is indicatedto help orient the various parts of the die, wherein the X-Z plane isdeemed to pass through the slot formed by the coating lips. The Y-Z andX-Y planes are respectively perpendicular thereto per the typicalcoordinate system orientation.

Referring now to FIG. 1 b, X, Y, Z coordinate system 23 of FIG. 1 b andassociated planes formed thereby is now described in more detail. X andY coordinates form X-Y plane 23.1, Y-Z coordinates for Y-Z plane 23.2.X-Z coordinates for X-Z plane 23.3. Hereinafter, an X-Y plane bending isdeemed to be a bending in the X-Y plane from a flatness 23.1.a to a bend23.1.b; an Y-Z plane bending is deemed to be a bending in the Y-Z planefrom a flatness 23.2.a to a bend 23.2.b; and a X-Z plane bending isdeemed to be a bending in the X-Z plane from a flatness 23.3.a to a bend23.3.b. The present invention focuses on the X-Z and Y-Z bending modes.

Not all dies need to be compensated. Dies which are long and thin in atleast one dimension will have a tendency to bend in the long planes.Defining a die by its width (or Z-dimension distance 10 in FIG. 1 a) toX-dimension distance 12 in FIG. 1 a) ratio and/or its width (orZ-dimension distance 10 in FIG. 1 a) to Y-dimension distance 14 in FIG.1 a) ratio will characterize the dimensional tendency for bending to besignificant in the X-Z and/or Y-Z plane, respectively. Generally dieswith the ratio equal to or greater than 2.5 will be considered forcompensation in accordance with the present invention. These dimensionsare the typical dimension of the structurally important portions of thedie. If it is difficult to state a “typical” dimension, then the averagedimension should be utilized. This is a geometric consideration. Squaresand cubes (e.g., ratio or ratios=1) do not tend to bend much due torestraining stiffnesses. As thermo-physical properties improve, theoptimizing job at any given ratio becomes easier. Improved propertiesfor steady state operation include increasing thermal conductivity (wattper meter per degree-Celsius) and reducing coefficient of thermalexpansion (meter per meter per degree-Celsius).

If properly designed and adjusted, the die will distribute the liquidevenly and uniformly out of the exit formed by the coating lips in athin layer. The present invention does not focus on the internaldistribution of the fluid in the die. Typically, the die can be adjustedradially to move toward or away from the substrate (in the X direction),thus determining the gap between the coating lips and the substrate,also referred to as the “coating gap.” For a given coating thickness,the flow parameters of the liquid can be determined, including the flowrate. Once these parameters are determined and the die is “set” in thecoating machine, the coating gap would typically be adjusted duringoperation. However, because of the extremely thin layers being coated,any such adjustments usually inject a certain degree of imprecision intothe process. There are also physical limitations on the accuracy of thedie itself. For example, it is very difficult to hold extremely smalltolerances on the lip geometries of the die, especially over the widthof the slot which may vary between a few and a hundred or more inches.

Referring now to both FIGS. 1 a and 2 a, there is illustratedschematically a typical die coating operation. Die 20 is shownpositioned adjacent to moving substrate 22 traveling in the Y direction24 in the area of coating lips 36 a, 36 b. Die 20 is shown simplifiedwithout heaters, insulation, or temperature sensors which are typicallyincluded with a fully operational die, but are described in more detailhereinafter in accordance with exemplary embodiments of the presentinvention. Substrate 22 travels around a back-up roll 26 as it passesacross the distal end of die 20. As shown in FIG. 1, it will beunderstood that both die 20 and the substrate 22 have substantiallyequal widths (in the Z direction), such that most of the entire width ofthe substrate is coated in one pass by the fluid 23 flowing into and outof the die and onto substrate 22.

Die 20 is modular in that it can be assembled from a number ofindividual elements and then set in the coater machine (i.e., a diestation, not shown) as a mountable device. Each die element may includefluid manifold 19 and a more distal die section 21. The most distalportion of the die section is referred to as coating lips 29, describedand illustrated in more detail in connection with FIGS. 2 a, 2 b.

Die 20 can be moved radially into or away from the back-up roll 26 inorder to adjust coating gap 30, which is defined as the distance betweencoating lips 29 and substrate 22. The elements of die 20 are separatedfrom each other slightly by a slot or feed gap 32 which allows thecoating material, i.e., fluid 23, to flow from fluid manifold 19 throughfeed gap 32 onto moving substrate 22.

Referring to FIG. 2 a, there is shown a close-up cross-sectionalschematic view taken in an X-Y plane 23.1 through a pair of coating lips36 a, 36 b positioned adjacent to moving substrate 22 to form coatinggap 30. It will be noted with respect to FIG. 1 a that substrate 22 inFIG. 2 a is shown to be flat or horizontal, whereas it actually willexhibit some curvature as it conforms to back-up roll 26. However, theconfiguration shown in FIG. 2 a is a good approximation of the fluidmechanics occurring in bead 42 of liquid formed in the coating gap 30between coating lips 36 a, 36 b and moving substrate 22.

Coating gap 30 is shown as dimension A in FIG. 2 a. It will beunderstood, that coating gap 30 can vary along the die width in the zdirection in accordance with different lip geometries, lip machiningdefects, angled or beveled lips, adjustments, misalignment, etc.

Referring to FIG. 2 b, there is shown a close-up cross-sectional view ofa multilayer die 21 which may be also utilized in accordance with thepresent invention. Although similar to die 20 in FIG. 2 a, die 21includes upstream and downstream die sections 50 a and 50 b, as well asa middle section 50 c separating the two. Formed between these varioussections are an upstream feed gap 52 a and a downstream feed gap 52 b.The liquid from upstream feed gap 52 a flows onto the substrate 22 toform a bottom layer 58, while the liquid from the downstream feed gap 52b flows onto the bottom layer to form a top layer 56.

The coating gap between the lip face and the substrate becomes criticalin providing a uniform layer onto the substrate. Because of the natureof the material of the die, e.g., steel and its operational temperaturestate, heating a die above atmosphere temperature, unless compensated,will cause non-uniform distortion of a steel die to occur and thecoating gap to become uneven over the die and substrate widths.

Heat distribution during a coating operation utilizing a knowngeometrical shape die can be thermally modeled. Referring briefly toFIGS. 8 and 9 temperature distributions can be color displayed utilizingspecific computer generated thermal modeling techniques, the colordisplay typically spanning from a hot area (e.g., in practice a red orwhite color, but indicated in FIGS. 8 and 9 by a higher number in therange of 1-12 temperature segments) to a cold area (e.g., in practice ablue color, but indicated in FIGS. 8 and 9 by a lower number in therange of 1-12 temperature segments). These operational temperaturegradients can result from the geometry of the die, the material of thedie, the location of heaters of a heating system applied to the die andthe accuracy/placement of temperature sensors to control such heaters.Because of the resulting differences in temperature, different parts ofthe die will expand or contract by different amounts, causing diedistortions.

Therefore, a key issue addressed by the present invention is how flatand parallel the lip face is to the substrate across width in the X-Zand Y-Z planes during die operation. Typical single layer dies canprovide for one of the pair of coating lips to be a fixed lip sectionand the other one of the pair of coating lips to be a flexible lipsection. The flexible lip section can be mechanically adjusted toprovide some assistance to help compensate for small magnitudes of feedgap unevenness. Heat distribution of an assembly of such a fixed/flexdie will be such that the fixed and flexible lip portions may distort indifferent directions with respect to the substrate. This may also occurin a fixed/fixed die. Also, when coating starts, the die begins to beheated differently because the fluid begins to interact with the heatingsystem. This can cause a change in bending state.

By simulating heat addition and loss in the correct amounts in thecorrect places to the thermal modeled die to remove the temperaturegradients, the structural model will verify a thermally corrected dieprior to die manufacture which provides a good approximation of a diewith a uniform coating gap.

Referring to FIG. 3, a design process flow in accordance with thepresent invention is established to provide for developing a thermallycorrected die which provides such a uniform coating gap. First, amanufacturing plant process need (e.g., for a hot melt die that doesn'tbend to distort the coating gap) is established (60). Next, current diescan be (optionally) analyzed, measuring bending under temperature, andproviding computer generated models to explain why the bending occurs(62). Then, models are used to create a die (64) that doesn't bend,using the models to show why a die bends by understanding thetemperature physics and making compensations therefor to meet processoperation objectives (e.g., non-bending die) and parameters (e.g., lipsstyle, size, shape, material). The models provide a basic configurationto start the compensation study process. Next, sensitivity studies arerun (66), taking into consideration environmental conditions, type ofinsulation needed to control heat loss, adhesive flow and airflowpatterns around the die. The sensitivity study develops an operatingenvelope for the die (68), which if not acceptable the die configurationgets adjusted (70) to meet the operating envelope. Once the operatingenvelope is found acceptable, the details as to die geometry,heater/temperature sensor/insulation types, size and locations areestablished for the contemplated die (72). A die is then fabricated (74)and inspected (76) hot and cold in accordance with standard drafting andmanufacturing processes. If the inspection proves successful the die canbe implemented for operation (78).

Referring to FIG. 4, the “develop new design concepts” (64) of FIG. 3 isset forth in further detail. First, current dies can be (optionally)checked and analyzed (80). Then, if current designs do not meet desiredcoating gap objectives a new and/or improved heating system design isconsidered (82), taking into consideration design requirements andlimitations (e.g., die size/shape geometrical window in which the diewill operate, material properties) and available technology. Thisresults in a preliminary design (84). Once the preliminary design isestablished, a three dimensional model (88) of the die components (e.g.,top half, bottom half, coating lips), which can influence heat transferin the die and its thermal map (temperature vs. spatial location), iscreated (86). From the solid model, a finite element meshing routine isused to create a mesh for the structural model (90) and a mesh for theheat transfer model (92). The parts (top, bottom, lips) are joined (94)and the heat transfer model is run (96). The temperatures are thenmapped onto the solid structural model (98) to determine the resultingdeformation.

Referring now to FIG. 5, the “run-sensitivity-studies” (66) of FIG. 3 isset forth in further detail. First, possible environmental conditionsand design issues are determined (100). Then, boundary conditions areset (102), such as amount airflow around the die, amount of fluidflowing. The boundary conditions sets are then applied to the thermalmodels (104). The thermal models are then solved (106) providing athree-dimensional temperature map of each condition (108).

The temperature map is then mapped onto a structural model (110). Thestructural model is then solved to determine the deformation (112). Theresults are then analyzed (114) to determine if the design isacceptable.

Referring now to FIG. 6, an example of a die bending magnitudecomparison for a progression of different die and die heating thermalzone and geometry configurations (each having heaters and temperaturesensors, with internal or external wirecages) is depicted. Thisillustrative study uses a simple criteria (with or without maximumpractical surface insulation) to test the thermal-dimensional stabilityof the design progression. The outer lines along the FIG. 6 x-axis arereference lines showing the thickness of a typical coating, e.g., +/−20microns. The inner dotted lines along the x-axis are also additionalreference lines showing machining tolerance for die cold, e.g., the lipface to be ground flat to be within 8-10 microns. The cross-hatched barsdepict the bending of the coating lips in the X-Z plane. The dotted barsdepict the bending of the die lip in the Y-Z plane. This data is helpfulto determine needed changes in the heating configuration and diegeometry, such as possibly needing to add heaters to certain locationsin the die, and determining temperature sensor locations.

Referring to FIGS. 7-9, an exemplative die design involving a fixed tophot melt slot die to minimize bending at the coating lips face inaccordance with the present inventive process is set forth. In FIG. 7there is depicted solid model 200 with heaters in place. The heaters aremodeled as rectangular slots to simplify the model. Heaters 202 a-202 drun longitudinal across the width of the die. Heaters 204 a-204 r runback to front, the front having the coating lips, partially across thedie.

To develop the appropriate heat distribution to meet die designobjectives, temperatures in a number of representative cross-sections ofthe die are examined. Referring now to FIG. 8, there is depicted athermal map with temperature legend of a representative cross-section ofa fixed top die 210, having coating lips 212, front wall 213, front walltaper 215, and a pair of front to back heaters 214 a, 214 b. Fluidmanifold 216 provides fluid to coating gap 218. Temperaturedistributions are depicted as spanning a 50° F. range from thermal areasidentified as 12 (maximum heat) to 6 (medium heat) to 1 (minimum heat)and therebetween. As can be seen, the temperature gradient is cool inrear area 220, hot in the areas near heaters 214 a, 214 b and somewhatmedium heat at coating lips 212. Therefore, with such a dieconfiguration the rear will tend to contract and the front will tend toexpand and the die will tend to bend concaved toward the back.

Referring to FIG. 9, when die 210 of FIG. 8, for example, has it'sgeometry adjusted and longitudinal heaters added a thermal map of animproved fixed top die 230 will result. Fixed top die 230 has coatinglips 232, front wall 234, front wall taper 236, a pair of front to backheaters 238 a, 238 b, and three longitudinal heaters 240 a, 240 b and240 c. Fluid manifold 242 provides fluid to coating gap 244. As in FIG.8, thermal gradients are depicted as spanning a 50° F. range fromthermal areas identified as 12 (maximum heat) to 6 (medium heat) to 1(minimum heat) and therebetween. As can be seen the temperature isuniform and the gradient is small throughout most of the die which willhelp prevent undesirable bending. This is a result of both geometrydesign changes, i.e., wire cages moved external to the die, dieshortened, unneeded material removed, the addition of longitudinalheaters, 240 a, 240 b and 240 c, and with the movement of front to backheaters 238 a, 238 b of die 230 being moved closer to the die exteriorthan that of front to back heaters 214 a, 214 b of die 210.

Referring now to FIGS. 10-15 b, exemplary embodiments of die apparatusand their heating system developed in accordance with the presentinvention is now described in more detail.

The heating system for the die is typically composed of heat sources(electrical resistance heaters, oil, steam, or other types of heatingand cooling sources), temperature sensors (such as thermocouples,resistance temperature detectors, thermistors, or other types oftemperature sensors), and thermal insulation and isolation materials,electrical interconnection hardware (if electrical heat is used and forsensors signals), fluid distribution devices (if oil, steam or otherfluids are used), etc. The heating system is developed concurrently withthe die geometry to gain maximum benefit from both.

As an example, the operating criteria for a Tool Steel die (such as AISIP-20 Tool Steel) and its heating system can include:

(1) To operate in a manner which maintains the X-Z bending flatness ofthe die lips to less than 0.001″ flatness deviation, preferably lessthan 0.0005″ deviation. This is as measured with a mechanical or opticalgage on a precision granite table.

(2) To operate in such a manner which maintains the Y-Z flatness of thelip faces to less than 0.004″ flatness deviation, preferably to lessthan 0.002′ deviation. This is as measured with a mechanical or opticalgage on a precision granite table.

(3) To not change the magnitude of lip flatness deviation in the X-Z orY-Z planes more than 0.001″ when coating commences, preferably less than0.0004″. This is as demonstrated by finite element modeling or othermeans.

(4) To allow controlled bending of the die at least in the X-Z plane.Bending shall be 0.0005″-0.003″ per 1° F. offset between rear and frontof the die (starting from the flat state of point 1) in the X-Z planefor the unconstrained die of the approximate configuration describedhere. This is as measured with a mechanical or optical gage on aprecision granite table.

(5) To maintain cross-width temperature deviation in the slot of lessthan 15° F., preferably less than 8° F., with the adhesive temperatureat or near the nominal temperature of the die. This is as determined byfinite element modeling, and verified with surface temperaturemeasurements at or near the lip faces.

The operating criteria for a tool steel die in the preceding paragraphsare applicable to a die with a width to X-dimension distance ratio up to11 or a width to Y-dimension distance ratio up to 14 and a steady stateoperating temperature of up to 200° C. In all cases, any width toX-dimension distance ratio and/or width to Y-dimension ratio greaterthan 2.5 and steady state operating temperatures greater than 200° C.are also possible, but the achievable requirements may change. Other dieconfigurations can be designed, but achievable requirements may change.Other materials may be considered, but the achievable requirements maychange based upon the thermal and physical properties of the material.Other means of determining flatness may be used, including strain gages,or other stress/strain measurement techniques. All of these changes canbe considered within the methodology outlined.

The die heating system heaters described herein are classified as“cross-sectional”, “cross-width”, or both. Cross-sectional heaters arethose heaters which have a substantial effect on the X-Z and Y-Zflatness or bending. Cross-width heaters are those heaters which arehave a substantial effect on the temperature distribution across thewidth of the die (Z-direction). Heaters can be both cross-sectional andcross-width. Heater refers to any active heat (or cooling) source. Theseare collectively referred to as active heat transfer means.

X-Z flatness is the most critical, since it directly translates into thethickness of the coating. Y-Z flatness is less critical as long as thesubstrate is close to flat relative to the size of the feed gap (i.e.,coating on a large diameter roll, i.e. 16″ with a small feed gap, i.e.,0.020″ is an approximately flat surface). Normally, the die is optimizedfor X-Z plane bending, then checked in the Y-Z plane for acceptability;though the Y - Z plane bending state is explicitly optimized in thedesign methodology. Cross-width temperature variability is critical tothe rheology of the fluid, but with respect to the present invention, itis considered mainly in relation to its interaction with thecross-sectional portion of the heating system.

FIG. 10 shows schematically simplified die 300 with heaters. Die 300includes fluid inlet 301 which communicates with internal longitudinalfluid trough coupled to fluid inlet 301 in a T-shaped manner (not shown)to allow the fluid to be dispensed across the width of the die. Fluidtrough opening 303 at the width extremes are capped/gasketed (not shown)to prevent fluid emerging from the respective width ends of the die.

In order to maintain bending flatness in the X-Z plane, heaters areinserted into cavities in the front end 305 and/or rear end 307 of thedie. Heaters that heat the front and rear of the die are deemed frontcross-sectional heaters and rear cross-sectional heaters, though in someinstances they may also function as cross-width heaters. The two heaters302, 304 are front heaters, each being a single heater runninglongitudinally through the width of the die. These heaters eachtypically have a single associated temperature sensor for feed back toregulate their power. These heaters could be made cross-width by placingseparately controlled zones within them, or by replacing the singleheater with multiple small individual heaters grouped into multiplezones, and in either case, adding a temperature sensor and control loopfor each zone.

Heater 306 and optional heater 308 are rear cross-sectional heaters.They are analogous to front heaters 302, 304 and also run longitudinallythrough the width of the die.

Heater groups 310, 312 are a plurality of individual heaters groupedinto separate cross-width die zones 314, 316, 318, 320, 322 and may beinserted into cavities into the front and/or rear of the die. In thedepicted embodiment of FIG. 10, heater groups 310 and 312 are insertedinto cavities in the rear of the die. Being cross-width heaters, and inthis case due to their length and placement, they mainly affect the rearof the die. As such, they can also be considered cross-sectionalheaters. A temperature sensor associated with cross-width heaters in aparticular cross-width die zone is placed in a location as to be moresensitive to the rear heating than the front heating in order to assurethis effect.

In the case of the cross-width heaters for this simplified example, thezoning is such that there are independently controlled zones 314, 322for the ends (to minimize end losses), independently controlled zone 318for the center (to accommodate fluctuating fluid inlet temperatures, andindependently controlled main heater zones 316, 320 (between ends andcenter). In one embodiment, die top half 324, while structurallyattached, is zoned independent of die bottom half 326 for morecross-sectional (Y-Z) flatness control.

Simplified FIGS. 11 a-11 e are shown in a representative X-Y planecross-section to describe the interrelation between cross-sectional andcross-width heaters. In these figures, the front and rear heaters arelongitudinal heaters, and the cross-width heaters are appropriatelyzoned individual heaters, with a single cross-width heater from therespective groups being shown. Typical die rear to front distance is5-10″. Typical die thickness of top or bottom is 2-4″.

FIG. 11 a illustrates a die which has two front longitudinal heaters302, 304, one rear longitudinal heater 306, and cross-width heaters 310,312 which are also cross-sectional heaters (rear heating). FIG. 11 bshows a non-typical but possible configuration where cross-width heaters330, 332 are also front cross-sectional heaters. This is generally notdone, but is possible. FIG. 11 c illustrates a situation similar to FIG.11 a, except cross-width heaters 334, 336 significantly extend acrossthe back to front direction of the die. In this case, the cross-widthheaters usually do not act as cross-section heaters. FIG. 11 d is a diewith no specific cross-width heaters, though as mentioned previously,independent zones could be manufactured into the longitudinal heaters.

All the dies illustrated in FIGS. 11 a-11 d are referred to as fixed topdies, meaning the feed gap is fixed and determined by machining of thedie halves. Referring now to FIG. 11 e, the die shown is similar to thedie depicted in FIG. 11 a, but with section 338 of metal cut out of thefront top half to form a flex top section. Various mechanisms could beput in place to allow the flex top section to be bent locally to modifythe feed gap. From a thermal perspective, in FIGS. 11 a-11 d, there isfree heat flow between the front and rear (as well as top and bottom) ofthe dies. In FIG. 11 e, there is free communication between the frontand rear of the bottom half, but not in the top half. The front of thetop half is effectively partitioned from the rear of the top half. Thereis some heat flow, but it is limited by the thickness of steel in flexsection 338. In general, for the exemplary embodiments of FIGS. 11 a and11 e, the cross-width heaters are rear cross-sectional heaters.

Referring now to FIG. 12, a portion of zone 316 of die bottom half 326and representative three heaters 312 a, 312 b, 312 c of the heater group312 of FIG. 10, is shown in conjunction with main zone temperaturesensor 400 and related heating and control system 402.

Each zone has a zone temperature sensor associated with the heaters(both cross-width heaters and cross-sectional heaters) in the particularzone. The sensor sends sensed temperature data to a proportionalintegral derivative (PID) temperature controller which compares thesensed temperature data with a temperature set point. If the comparisonshows that the sensed temperature is below the temperature set point,the PID controller will signal the heaters to increase power. If thecomparison shows that the sensed temperature is greater than thetemperature set point, the PID controller will signal the heaters todecrease power. Accordingly, these zone temperature sensors are used ineffect to control the die cross-width temperature. Each zone temperaturesensor is located in the die in a centralized proximity to the heatersto be controlled in its zone.

Referring again to FIG. 12, main zone temperature sensor 400, which inone embodiment can be a resistance temperature detector (RTD) withsensing tip 410 installed in a tube which is inserted and supported at aconvenient orientation within die bottom half 326 through a preformedsensor channel or drilled hole in the die. The sensor is located to meetpredetermined temperature criteria. These locations can varyaccordingly. For example, in the design methodology, temperature sensorlocations (for a tool steel die with width to X-dimension distance ratioand/or width to Y-dimension distance ratio greater than 2.5, operatingat least up to 200° C. in room air) could be chosen for each zone whichmeet the following conditions as determined by finite element modeling:

(1) the sensing tip of all temperature sensors (all zones) are within 1°F., preferably less than 0.2° F. of the nominal die temperature, whichis within 10° F., and preferably 2° F. of the entering fluidtemperature. Condition (1) is met when fluid is not flowing through thedie (for visually setting the coating gap, measuring flatness, etc.).Condition (1) is met while fluid within 2° F., and up to 10° F. of thenominal die temperature is flowing through the die at up to maximum flowrate.

(2) local temperature gradients in the region of the sensing tip, wherepossible, are less than 5° F. per inch, and preferably less than 1° F.per inch.

The location of heaters and sensors within a specific die geometry aresuch that these requirements are met while meeting thepreviously-mentioned requirements (1)-(5), possibly after smalltemperature offsets are determined by measurement. Sensor 400 reads thedie temperature 406 at its location and provides the sensed temperaturedata to temperature controller 404 which has a predetermined desiredzone temperature set point 408. Temperature controller 404 performs acomparison between the measured die temperature and the set pointtemperature and sends temperature differential control signal 420 toheater control 422, such as a relay mechanism which allows current toflow to respective resistive heaters. When the sensed die temperatureand set point temperature are the same current flow to the heatersremains constant.

In addition there can be front top, front bottom, rear top and rearbottom temperatures sensors meeting the above-criteria. These sensorsare used to control the die cross-section temperature. These sensorswould be located in the die in a centralized proximity to thecross-section heaters in the respective front-top, front bottom, reartop and rear bottom longitudinal cross-section heaters.

Referring now to FIG. 13, as an example of cross-section heaters andtheir heater control, a portion of the end of die bottom half 326 and arepresentative three heaters 304, 306 of FIG. 10, is shown inconjunction with their respective longitudinal temperature sensors 450,452 and related respective heating and control circuitry 454, 456. Fluidtrough opening 303 is shown exposed in FIG. 13 but would normally besealed as described above. Each cross-section heater has its own sensorand related heating and control system. For example, front bottomcross-section heater 304 extends across the width of die bottom 326 andhas an associated front bottom temperature sensor 450 which is coupledto front bottom heating and control system 460. Heating and controlsystem 460 includes temperature control 462 and heater control 464.Similarly, rear bottom cross-section heater 306 extends across the widthof bottom die 326 and has an associated rear bottom temperature sensor452 which is coupled to rear bottom heating and control system 454.Heating and temperature control system 454 includes temperature control456 and heater control 458. Both front bottom heating and control system460 and rear bottom heating and control system 454 operate in a similarmanner to that previously disclosed with regard to heating and controlsystem 402 of FIG. 10. The sensors 450 and 452 are located in a locationdetermined by finite element modeling to meet the previous criteria,e.g., along the length of the heater in proximity to their respectiveheaters 304, 306, typically 0.5″ from their respective heater, such thattheir associated set points can provide for controlling the front andback temperatures to be the same.

Each of the sensors, whether sensors for cross-section heaters orcross-width heaters are also located also that cross-talk from othersensed areas is minimized, whether associated with other cross-sectionheating areas or other cross-width heating zones.

Each of the respective heating and control systems for both thecross-width and the cross-section heaters will then cycle theirrespective system feedbacks such that all the sensors across the entiredie are at the same temperature. Once heater and sensor locations areproperly chosen for a given die geometry with all expected attachmentsand heat losses, and all the die areas are at the same temperature, thedie can be deemed flat for the stipulated operating conditions.

Referring now to FIG. 14, a partial portion of the die shown in FIG. 10having a top die portion 324 and bottom die portion 326 is shown withfurther attachments which may affect the overall temperaturedistribution in the die. Top die portion 324 has affixed to the rearportion thereof wirecage 500 which can collect the various cross-widthheater wirings associated with the top die portion. Similarly, bottomdie portion 326 has affixed to the rear portion thereof wirecage 502which can collect the various cross-width heater wirings associated withthe bottom die portion. The end of the die includes gasket plate 504 toseal the end of the fluid trough. Top and bottom die cross-sectionaland/or cross-width sensor wirings can run longitudinally (such as in aformed die channel) and terminate in respective connector housings 506,508. Mounting blocks, such as block 510 can be coupled to the die andallow the die assembly to mountably sit into a die station housingstructure adjacent to the die (not shown).

However, it should be understood that the present top and bottom diecombination may be thermally isolated from any such die station housingadjacent to the die. As such, the present inventive method and apparatusis directed to an integrated main heating system for the heated die andis not concerned primarily with the heat loss associated with the diestation housing. Preferably, any heating of the fluid to bring it to itsproper temperature for application on the substrate is done separatelyfrom the heaters of the present invention as much as possible. Needlessto say, the heat convected by the heated fluid as it passes through thedie from fluid inlet and out through the coating lips will effect theoverall die temperature distribution.

Referring now to FIGS. 15 a and 15 b (which is a blow-up of a portion ofFIG. 15 a), there is shown schematically in simplified cross-section, afurther embodiment of the present invention. As can be seen, substrate22 travels in Y direction 24 as back-up roll 26 rotates in direction 25.Die 300 includes thermal insulators 600, 602 on the top and bottomsurfaces of die 300 and wind guard 604 which deflects in direction 606wind produced by rotation of back-up roll 26. The air flow guardprotects the front lower portion of the die from localized cooling dueto stripping of an air boundary layer from the substrate. Area 608 isdepicted in FIG. 15 b. Also seen are wirecages 500, 502 and mountingblock 510 fitting into die station housing 512. The modeling process inaccordance with the present invention takes into account the variousfeatures of the die and these attachments thereto which can affect thetemperature distribution within the die.

The front cross-section heaters generally provide 20% -60% of the totalapplied heating power of the die, depending on actual heater placementand sensor locations. In the exemplary configuration, they tend to run25% -45%. This is based on actual power output of the temperaturecontrollers. Rear heaters (plus any optional small auxiliary heaters)run most of the balance (including cross-width heaters). Front and rearlongitudinal heaters generally are located such that their centers areless than 1.5″ from the outside surface closest to them. Cross-widthheaters generally start from the rear of the die and extend towards thefront 3-6″.

These configurations are the exemplary embodiments since most of thevarious items attached to the die tend to be attached at the rear,leading to more heat loss variation there. End heaters can be placedsuch that they heat the entire end.

In the exemplary embodiments, wire wound platinum resistance temperaturedetectors (RTDs) pre-screened for an accuracy of better than +/−1° F.,or preferably better than 0.4° F. at a target temperature (i.e., 346°F.) are used. In this case, replacement will not significantly effectthe bending state of the die. Also, drift of wire wound platinum RTD'sare known to be very small over time. High accuracy PID temperaturecontrollers could include, but not be limited to Syscon RKC SR Mini HGSystem.

A further exemplary implementation could be as follows:

(1) Operate the front heaters such that they (and thus the slot andmanifold) are at nominal temperature. The coating will enter the die ator near this nominal temperature.

(2) In actual measurements of flatness after the die is manufactured, ifany offsets to set points for cross-sectional zones in the heatingsystem (due to uncertainties in the finite element modeling which led toheater and sensor locations) are needed to bring the die to within themeasured flatness specification, make them at the rear of the die ifpossible. This is so: (a) the temperatures of the slot and manifold,which are dominated by the front heaters, will stay at the temperatureof the adhesive and thus corrections to bending will have minimal effecton the rheology of the fluid; and (b) Since the fluid typically entersat the center of the rear of the die, the heat transfer to a higher orlower temperature will be minimal since the thermal conductivity ofmaterials coated in these dies is typically very low, and the distancefrom the metal wall to center of the tube is typically significant;conversely in the front of the die, the fluid is spread to a thin film(typically less than 0.060″) with a large heat transfer area, thus heattransfer rates are much higher.

(3) These offsets may be necessary because: (a) the thermal andstructural finite element models have uncertainties in them. This canlead to uncertainty in the cross-sectional and/or cross-widthtemperature distribution. Coupling this with using a single temperaturesensor to establish a temperature for a single zone (i.e., the rearzone), can lead to a shift in actual temperature measured at the sensorfrom what was originally predicted. It has been demonstrated that oncethe shift is corrected, the die is stable. (b) Attachments to the die,such as metallic wire cages (to house heater and/or sensor wires) caninteract with the die and each-other in complex ways, often difficult tomodel correctly (especially in terms of radiation heat transfer andairflow modification), adding to uncertainty in local temperaturedistributions near the attachment points. (c) Depending on how the dieis machined and stress relieved, direction-preferential stresses mayexist which are not easily accounted for by finite element modeling. Theoffset between the maximum and minimum cross-section heating zonesetpoints should not exceed 10° C., and preferably should not exceed 4°C.

(4) Insulate the die as much as possible from attachments and isolate itfrom any external mounting structure to minimize the existence of localhot or cold spots, which complicate the ability to accurately predicttemperature distributions, leading to uncertainties in choosing heaterand sensor locations. Also, consider insulating large surfaces tominimize heat loss by convection to air, and reduce radiation heatlosses; thus reducing temperature gradients near these surfaces. Wherepossible, place an insulating layer between die surfaces and any metalattachment. Where the die is mounted to its support structure, usestructural insulating materials to isolate the die from heat loss to themounting structure. Use of insulation on large surfaces can be helpfulin minimizing sensitivity to environmental conditions. Use some type ofshield to the front bottom (and optionally top) of the die to deflecthigh velocity air carried by the substrate from cooling the front of thedie. FIGS. 15 a and 15 b shows such a shield (wind guard), plus apossible insulating strategy. Overall, minimize the number and magnitudeof effects that a given heater zone needs to accommodate (i.e.,convection heat loss, attachment heat loss, interaction with fluid,etc.). The more effects, and the larger the magnitude, the greater thepossibility of compromise and sensitivity to different operatingconditions. Isolate the die from heat loss when mounting it. This meansthe heating system only has to deal with losses to the atmosphere, andlimited losses to attachments.

Those skilled in the art can appreciate that alternative embodiments tothose described and shown in the figures can fall within the scope ofthe present invention. For example, referring back to FIG. 2 b, thoseskilled in the art can appreciate that the inventive concepts describedin conjunction with FIGS. 10-15 b can be applied to the multiple sectiondie in FIG. 2 b. Cross-section heaters, cross-width heaters and theirassociated sensors can be modeled and located as appropriate for diesections 50 a, 52 a and 52 b of FIG. 2 b.

1. A coating die apparatus comprising: a die having a Z-dimension diewidth distance to X-dimension die distance and/or Z-dimension die widthdistance to Y-dimension die distance ratio greater than 2.5, and amechanically suitable operating temperature, die lip faces in a Y-Zplane and a slot opening perpendicular to the lip faces in an X-Z plane,the dip lip faces forming the slot opening at a front portion distalfrom the rear portion, a coating gap being formed between the die lipfaces and a substrate upon which a fluid layer is applied onto thesubstrate from the slot opening across the Z -dimension die widthdistance; and an integrated heating system having: one or more frontcross-section heaters spaced within the die longitudinally across thewidth and proximate to the front portion, each front cross-sectionheater, being optionally a top front cross-section heater or a bottomfront cross-section heater, having a respective front cross-sectiontemperature sensor, each front cross-section temperature sensor beingcoupled to a front cross-section temperature control system to regulatethe heat being applied by the respective front cross-section heaters;and one or more rear cross-section heaters spaced within the dielongitudinally across the width and distal to the front portion, eachrear cross-section heater, being optionally a top rear cross-sectionheater or a bottom rear cross-section heater, having a respective rearcross-section temperature sensor, each rear cross-section temperaturesensor being coupled to a rear cross-section temperature control systemto regulate the heat being applied by the respective rear cross-sectionheaters; wherein with the integrated heated system operating at a steadystate operating temperature, having a difference between a maximum crosssectional temperature setting and a minimum cross section temperaturesetting equal to or less than a resultant temperature difference due touncertainties in die design optimization processing, and the front crosssection heaters supplying a characteristic portion of total powerrequired to maintain the steady state operating temperature and the rearcross section heaters and/or other heaters supplying the remainder oftotal power, the integrated heating system: being balanced to minimizetemperature gradients along the width in the X-Y plane for maintainingbending flatness of the coating lip faces in the X-Z plane to less thana defined flatness deviation while not changing the magnitude of die lipflatness deviation in X-Z plane or Y-Z planes more than a definedflatness deviation when coating commences; and being optionallyunbalanced by changing: a temperature difference between frontcross-section heaters and rear cross-section heaters by modifying aratio of heating power applied to front cross-section heaters to heatingpower applied to rear cross-section heaters for controlling bending ofthe die in the X-Z plane; and/or a temperature difference between topcross-section heaters and bottom cross-section heaters by modifying aratio of heating power applied to top cross-section heaters to heatingpower applied to bottom cross-section heaters for controlling bending ofthe die in the Y-Z plane.
 2. The coating die apparatus of claim 1,wherein the Z-dimension die width distance to X-dimension die distanceratio is up to 11 and the Z-dimension die width distance to Y-dimensiondie distance ratio is up 14, the operating temperature is up to 200° C.,the material of construction is steel, the difference between themaximum cross sectional temperature setting and minimum cross sectiontemperature setting is equal to or less than 10° C., and the front crosssection heaters supplying a characteristic 20%-60% of the total powerrequired to maintain the steady state operating temperate and bendingflatness of the die lip faces in the X-Z plane to less than 0.001″flatness deviation, while not changing the magnitude of lip flatnessdeviation in the X-Z plane or Y-Z planes more than 0.001″ when coatingcommences, and having an ability to purposefully bend the die in the X-Zplane at a rate of 0.0005″ to 0.003″ per 1° F. of purposefully inducedtemperature gradient in the X-Z plane.
 3. The coating die apparatus ofclaim 2, wherein the balancing of the integrated heating system isoptimized to maintain bending flatness of the die lip faces in the X-Zplane to less than 0.0005″ flatness deviation, while not changing themagnitude of lip flatness deviation in the X-Z plane or Y-Z planes morethan 0.0004″ when coating commences.
 4. The coating die apparatus ofclaim 2, wherein the balancing of the integrated heating system isoptimized to maintain die bending flatness in the Y-Z planes to lessthan 0.004″ flatness deviation.
 5. The coating die apparatus of claim 2,wherein the integrated heating system further comprises groups ofcross-width heaters spaced within the die in back portion to frontportion direction and/or front portion to back portion direction, acrossa die width in zones, each zone having a respective cross-widthtemperature sensor, each cross-width temperature sensor being coupled toa respective cross-width temperature control system to regulate heatbeing applied by the respective cross-width heaters in the respectivezone.
 6. The coating die apparatus of claim 5, wherein cross-widthheater zones are thermally balanced simultaneously with cross-sectionalheater zones to maintain cross-width temperature deviation in the slotopening of less than 15° F., with the adhesive temperature being at thenominal temperature of the die, or within 10° F. thereof.
 7. The coatingdie apparatus of claim 4, wherein cross-width heater zones are thermallybalanced simultaneously with cross-sectional heater zones to maintainbending flatness in the Y-Z planes to less than 0.002″ flatnessdeviation.
 8. The coating die apparatus of claim 6, wherein cross-widthheater zones are thermally balanced simultaneously with cross-sectionalheater zones to maintain cross-width temperature deviation in the slotopening less than 8° F., with the adhesive temperature being at thenominal temperature of the die, or within 2° F. thereof.
 9. The coatingdie apparatus of claim 2, wherein the die apparatus is externallyinsulated to reduce or eliminate temperature gradients.
 10. The coatingdie apparatus of claim 2, wherein an air flow guard is located on thefront portion to protect the die from localized cooling due to strippingof an air boundary layer from the substrate.
 11. The coating dieapparatus of claim 2, wherein the front cross-section temperaturesensors and the rear cross-section temperature sensors are wire woundplatinum resistance temperature detectors having an accuracy of betterthan +/−1° F.
 12. The coating die apparatus of claim 2, wherein thecross-width temperature sensors are wire wound platinum resistancetemperature detectors having an accuracy of better than +/−1° F.
 13. Thecoating die apparatus of claim 1, wherein the integrated heating systemfurther comprises groups of cross-width heaters spaced within the die inback portion to front portion direction and/or front portion to backportion direction, across the width in zones, each zone having arespective cross-width temperature sensor, each cross-width temperaturesensor being coupled to a respective cross-width temperature controlsystem to regulate heat being applied by the respective cross-widthheaters in the respective zone.
 14. The coating die apparatus of claim1, wherein the die apparatus is externally insulated to reduce oreliminate temperature gradients.
 15. The coating die apparatus of claim1, wherein an air flow guard is located on the front portion to protectthe die from localized cooling due to stripping of an air boundary layerfrom the substrate.
 16. The coating die apparatus of claim 1, whereinthe front cross-section temperature sensors and the rear cross-sectiontemperature sensors are wire wound platinum resistance temperaturedetectors having an accuracy of better than +/−1° F.
 17. The coating dieapparatus of claim 1, wherein the cross-width temperature sensors arewire wound platinum resistance temperature detectors having an accuracyof better than +/−1° F.